JEE Advanced · 2012 · 08 Apr 2012 · Paper 1 · Coordination Compounds

An aqueous solution of copper sulfate (${\mathrm{CuSO}}_4$) appears blue because it absorbs light in the complementary color region. According to the color wheel, blue is complementary to orange-red. The ${\mathrm{Cu}}^{2+}$ ion in aqueous solution (as $[\mathrm{Cu}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$) absorbs orange-red light (around 600-700 nm) and transmits blue light, which is what we observe. Therefore, the color absorbed is orange-red.